(a) Field of the Invention
The present invention relates to a display and a driving method thereof, and more particularly to an organic electroluminescence (hereinafter, “EL”) display of an active matrix driving method.
(b) Description of the Related Art
In general, an organic EL display is a display that emits light by electrical excitation of fluorescent organic compound and displays image by driving each of M×N organic luminescent cells with voltage or current.
This organic cell has a structure of an anode (ITO), an organic thin film and, a cathode layer (metal). The organic thin film is formed as a multi-layered structure including an emission layer (“EML”), an electron transport layer (“ETL”), and a hole transport layer (“HTL”) so as to increase luminescence efficiency by balancing electron and hole concentrations. In addition, it can include an electron injection layer (“EIL”) and a hole injection layer (“HIL”) separately.
Organic EL displays that use organic luminescent cells like the above are configured as passive matrix or active matrix that includes thin film transistors (TFTs). In the passive matrix configuration, organic luminescent cells are formed between anodes and cathodes lines that cross each other and driven by driving those lines. While in the active matrix configuration, each organic luminescent cell is connected to a TFT usually through an ITO electrode and driven by controlling the gate voltage of the corresponding TFT.
FIG. 1 is a circuit diagram of a conventional pixel for driving the organic EL display using TFTs, and it is a representative of M×N pixels. Referring to FIG. 1, driving transistor Mb is connected to organic EL device OLED to supply current for emitting light. The amount of current through driving transistor Mb is controlled by data voltage applied through switching transistor Ma. In this case, capacitor C1 for maintaining the applied voltage during a certain period is connected between source and gate of transistor Mb. Scan line XM is connected to the gate of transistor Ma, and data line YN is connected to the source thereof.
Operation of the pixel is as follows. When switching transistor Ma is turned on by the selection signal applied to the gate thereof, a data voltage is applied to node A, the gate of the driving transistor through the data line. Then, a current corresponding to the data voltage applied to the gate thereof flows into the organic EL device OLED to emit light.
In this case, current IOLED flowing through organic EL device OLED is referred to as Equation 1.
                              I          OLED                =                                            β              2                        ⁢                                          (                                                      V                    GS                                    -                                      V                    TH                                                  )                            2                                =                                    β              2                        ⁢                                          (                                                      V                    DD                                    -                                      V                    DATA                                    -                                                                                V                      TH                                                                                          )                            2                                                          (        1        )            wherein, IOLED is a current flowing through organic EL device OLED, VGS is the gate-to-source voltage of transistor Mb, VTH is a threshold voltage of transistor Mb, VDATA is a data voltage and β is a constant.
As expressed in Equation 1, according to the pixel circuit of FIG. 1, the current corresponding to the applied data voltage is supplied to organic EL device OLED, and organic EL device OLED emits light in correspondence to the supplied current. Herein, the applied data voltage has many levels to express corresponding gray levels.
However, in the conventional pixel as described above, there is a problem in that high gray scale is difficult to obtain due to variation of the threshold voltage of TFTs generated by manufacture process. For example, when driving transistor Mb is supplied with data voltage in the range of 3 volts, two data voltages representing adjacent gray levels must be apart from each other by approximately 12 mV (=3V/256) so as to implement 8-bit (256) gray scale. If the threshold voltage varies in 100 mV range, which is usually the case, it is difficult to discriminate one data voltage from another and, as a result, gray scale is reduced.